This invention relates to steam turbines, and more particularly to a positive drain for associated power plant piping and vessels which must be drained of water to prevent possible damage to the turbine.
The number of incidents of water induction into steam turbines has demonstrated the importance of proper drainage of pipes and vessels in both the primary and secondary cycles of power plants. Certain extraction piping and feedwater heater bleed piping from the turbine should be provided with positive drains to prevent accumulation of water therein. If allowed to accumulate, the water may flow back into the turbine, or flash due to a reduction of pressure resulting from a loss of load on the generator, and cause water or cold steam to enter the turbine.
There are numerous methods of keeping turbine pipes and vessels free of unacceptable accumulations of water. First the water may be drained continuously by keeping a valve open during periods when water is expected to be present; or by use of a continuously open drain, orificed to minimize steam flow when water is not present. Second, the water may be drained after a predetermined amount accumulates in a level-controlled drain pot. This method utilizes a level switch or level controller which actuates a power operated valve located in a drain line. The drain line is the line which runs from the drain pot to a vessel which receives the drainage, such as a condenser. Third, the water may be prevented from accumulating through the use of heated pipes and vessels. This is done, for example, by discharging a continuous flow of hot steam through a small orifice in the vessel to a lower pressure zone. The orifice is sized to pass just enough steam to heat the vessel and prevent condensation.
In these methods, orifices are used to reduce the loss of high energy steam through drains. However, a major problem with orifices in drain lines is that they are easily plugged by debris present in the primary and secondary cycles of a power plant. This is especially true during initial startup, and during startups subsequent to maintenance involving disassembly of equipment. However, even when equipment is not disassembled, the products of wear, corrosion, and erosion, such as hard particle erosion can plug orifices. This problem can become acute in systems using small orifices in conjunction with high pressure sources, such as in the heated pipe method described above.
One attempt at relieving this problem was the development of a coarse strainer integral with an orifice and combined in a plug assembly. The plug assembly was installed in a block so that the orifice could be cleaned periodically. A disadvantage with such strainers is that they themselves may become blocked by debris, necessitating their removal, cleaning and rewelding into place. This tends to be expensive and time consuming, and therefore undesirable.
Another solution to the problem was a plug-resistant orifice assembly that could be cleaned by reversing the flow without disassembly. U.S. Pat. No. 3,792,719, assigned to Westinghouse Electric Corporation, was issued on a pair of valves incorporating two valve stems and capable of reversing the flow through a flow restriction.